WATER, AND ITS USES 99 



to either of the others. When diffusion of liquids or diffu- 

 sion of gases occurs through a porous (permeable) partition, 

 the phenomenon is called osmosis. 



To change a solid to the liquid state requires an expendi- 

 ture of heat. This is manifestly the case in the melting 

 of ice, and of such substances as sulphur or lead. As salt 

 dissolves in water there may be a noticeable fall in the 

 temperature of the liquid. Heat is abstracted from the 

 water to aid in changing the salt to a solution. When salt 

 has been scattered over ice that has been broken into small 

 lumps the surfaces of which are water covered, the lumps may 

 become frozen together. This is because the heat abstracted 

 from the ice cold water as the salt dissolves is sufficient to 

 convert the water into ice. In the freezing of ice cream the 

 heat lost by the cream is partly used in the solution of the 

 salt to form a brine, but more largely in the melting of the 

 ice packed around the can that contains the cream. 



The boiling temperature of the solution of any solid in 

 water is higher, and its freezing temperature is lower, than 

 with pure water. A saturated solution of common salt, for 

 example, does not freeze until the temperature reaches 

 -22 C. Advantage is taken of this fact in making ice 

 cream, and in the use of solutions of calcium chloride or of 

 common salt (sodium chloride) for the brine into which the 

 cans of pure water are set in making artificial ice. The 

 waters of sheltered inlets of the ocean may remain open in 

 winter, when near by fresh waters in rivers and lakes become 

 ice covered. 



Osmosis in plants and animals is a phenomenon of utmost 

 importance. The circulating fluid in both cases, containing 

 as it does the dissolved food material, must penetrate all 

 parts of the tissues and make available to every living 

 cell the nourishment it needs. 



The less dense any solution or gas is the more likely it is 



